The long-term objective of this research is to understand how neural activity modifies synaptic connectivity in brain and to understand the relationship between this altered connectivity and behavioral learning and memory. Using associative long-term potentiation (LTP) as a model for understanding how neuronal connectivity changes as the pattern of neural activity changes, the proposed studies address the question of altered existing axospinous synapses and synaptogenesis as correlates of LTP in the adult rat hippocampus, a brain structure that may figure prominently in mental health-related cognitive dysfunction. The system to be investigated is the excitatory entorhinal cortical (EC) projection to the CA1 pyramidal cell distal apical dendritic tuft in s. moleculare. Our preliminary studies indicate that the dendrites and synapses in adult CA1 s. moleculare l) share features with the axospinous synapses in the dentate gyrus, where the morphological correlates of LTP imply the modification of existing synapses; and 2) possess some unexpected morphological features that suggest ongoing dendritic growth and synaptogenesis. These features include: filopodia; large, varicose dendritic spines; a relatively high probability of spine-associated polyribosomes; invaginations of pyramidal cell dendrites by other dendrites and axons; and synapseless protrusions from dendrites. These morphological observations in normal adult CA1 s. moleculare lead to the hypothesis that the morphological correlates of LTP here will differ from those obtained previously in other hippocampal regions. Specific Aim l characterizes the anatomical correlates of LTP at the EC-CA1 synapses to test the hypothesis that LTP correlates with increased growth there. Specific Aim 2 characterizes the synaptology of the EC-CA1 synapses at the electron microscopic level and confirms that pyramidal cell dendrites participate in the unusual morphological interactions observed in CA1 s. moleculare. Specific Aim 3 tests the hypothesis that blocking the NMDA receptor, which prevents the induction of LTP in several hippocampal systems, also prevents the anatomical correlates of LTP. Specific Aims l and 3 principally use the in vitro hippocampal slice preparation, where the EC-CA1 pathway can be studied less ambiguously than in vivo, followed by: stereological determination of the number of synapses per pyramidal cell; quantification of functionally relevant synaptic features; and serial section evaluation and reconstruction of growth-associated morphological features. Using anterograde tracer injections into the EC to label EC-CA1 synapses at the ultrastructural level, Specific Aim 2 determines the types of postsynaptic elements with which EC axons synapse in CA1. The gold-toned rapid Golgi method is also used to characterize at the light and electron microscopic levels the growth-like structures identified in preliminary studies. The anatomical correlates of LTP obtained here, together with previous studies in other hippocampal systems, should allow future studies to evaluate structural changes in hippocampally relevant behavioral learning paradigms. In this way, additional insight into the relationship between structural modification and cognition may evolve. Because the hippocampus apparently plays a key role in cognition, understanding activity-dependent synaptic modification in the hippocampus should also help us to better understand hippocampal dysfunction and its impact on cognition.